Mitochondria play a central role in cell bioenergetics and control multiple aspects of neuronal life and death, including regulation of Ca2+ signaling. By buffering Ca2+ during excitation, and subsequently releasing Ca2+ back to the cytosol, mitochondria shape [Ca2+]i signals and regulate numerous Ca2+-dependent functions in neurons, such as excitability, synaptic plasticity and gene expression. However, excessive load of mitochondria with Ca2+ trigger neurotoxic processes in stroke and in Alzheimer's, Parkinson's and Huntington's diseases. In spite of the importance of neuronal mitochondrial Ca2+ transport, the molecules mediating mitochondrial Ca2+ uptake and release in neurons are not known. This knowledge gap presents a major obstacle in our progress toward understanding and therapeutically exploiting mitochondrial functions. Thus, the first objective of this proposal is to identify the molecular components of mitochondrial Ca2+ uptake in neurons. Mitochondria are highly dynamic organelles that rapidly undergo fission and fusion (MFF), which affects transport of mitochondria, synaptic plasticity and neuronal survival. Notably, mitochondrial fission is an early event in stroke, and fragmented mitochondria are prevalent in Alzheimer's and Huntington's disease. Given the central role of mitochondrial Ca2+ transport in neuronal life and death, it is possible that the effects of MFF on neuronal survival are mediated in part through changes in mitochondrial Ca2+ handling, although this idea has not been tested. Thus, our second objective is to determine how MFF status affects mitochondrial Ca2+ transport and Ca2+ homeostasis in neurons. Based on our preliminary data and published literature, we hypothesize that CCDC109A, CCDC109B and MICU1-3 are essential molecular components of mitochondrial Ca2+ uptake in neurons, and that the MFF process provides important control of CCDC109A and/or CCDC109B activities, mitochondrial Ca2+ transport and Ca2+ homeostasis in neurons exposed to neurotoxic conditions. We will use innovative approaches, including genetically encoded mitochondrial Ca2+ sensors, electron probe X-ray microanalysis and novel genetic mouse strains, to test our hypothesis in three specific aims. Aim 1 will identify the roles of novel proteins CCDC109A, CCDC109B and MICU1, 2 and 3 in mitochondrial Ca2+ uptake in neurons. Aim 2 will determine how mitochondrial restructuring regulates mitochondrial Ca2+ transport in neurons and examine specific roles of CCDC109A and CCDC109B phosphorylation in this process. Aim 3 will examine the function of MFF in maintaining neuronal Ca2+ homeostasis under neurotoxic conditions, such as excessive exposure to glutamate and ischemia. This project will provide insight into the molecular organization of mitochondrial Ca2+ transport in neurons and will establish mechanistic links between mitochondrial dynamics, Ca2+ signaling and neuronal Ca2+ homeostasis. We anticipate that these studies will be transformative because they will identify new molecular and genetic tools for exploring many functions of mitochondrial Ca2+ uptake in neurons and may lead to new therapeutics targeting mitochondrial Ca2+ transport and MFF for treating stroke and neurodegeneration.